1. Field of the Invention
The present invention relates to conformal coating of a thin polymer electrolyte layer on nanostructured electrode materials for three-dimensional micro/nanobattery applications.
2. Background of the Invention
The reversible electrochemistry and the superior gravimetric and volumetric energy storage capacities of lithium ion batteries (LIB) have propelled them as the dominant power source for a range of portable electronic and vehicular applications. [Armand, M. et al., Nature 2008, 451, 652-657; Whittingham, M. S., Chem. Rev. 2004, 104, 4271-4301; Van Schalkwijk, W., et al. Advances in Lithium-Ion Batteries (Kluwer Academic/Plenum, New York) 2002] But the sluggish rate of charge/discharge remains one of the major challenges plaguing the LIB industry to date. Current LIBs suffer slow rates of charge/discharge because of (i) slow diffusivity constants of Li ions in most of the electrode materials, (ii) large separator thickness of the order of 20 μm, and (iii) slow electron transport due to large micrometer sized electrode materials. A plethora of recent research efforts have tried addressing these drawbacks of the present day LIBs. [Taberna, P. L., et al., Nat. Mater. 2006, 5, 567-573 (“Taberna 2006”); Chung, S. Y., et al., Nat. Mater. 2002, 1, 123-128; Li, N. et al., Electrochem. Solid-State Lett. 2000, 3, 316-318; Aricò, A. S. et al., Nat. Mater. 2005, 4, 366-377 (“Aricò 2005”); Reddy, A. L. M. et al., Nano Lett. 2009, 9, 1002-1006 (“Reddy 2009”); Endo, M. et al., Carbon 1999, 38, 183-197; Lee, S-H. et al., Adv. Mater. 2008, 20, 3627-3632]
Shorter Li ion diffusion paths have been achieved by fabricating thin film electrodes. [Bates, J. B., et al., Solid State Ionics 1999, 135, 33-45] This has led to a significant increase in charge/discharge rates. Yet the thin film technology has the major drawback of possessing lower energy densities. Hence, current research has focused on assembling the entire batter (current collector, anode, solid polymer electrolyte, cathode) in the 3D nanostructured architecture and several designs have been proposed. [Long, J. W., et al., Chem. Rev. 2004, 104, 4463-4492 (“Long 2002”)] 3D design offers marked improvements in energy and power density especially with respect to the geometric foot print of the device. [Long 2002] Efficient means of energy storage with a smaller areal footprint has been the focus of many researchers in the recent past. [Golodnitsky, D., et al., Solid State Ionics 2006, 177, 26-32 (“Golodnitsky 2006”); Hassoun, J., et al., Adv. Mater. 2007, 19, 1632-1635 (“Hassoun 2007”); Nam, K. T., et al., Proc. Natl. Acad. Sci. USA 2008, 105, 17227-17231; Shaijumon, M. M., et al., Chem. Comm. 2008, 20, 2373-2375; Powers, R. A., Proc. IEEE 1995, 83, 687-693; Nathan, M., et al., J. Microelectromech Syst. 2005, 14, 879-885 (“Nathan 2005”); Cho, Y. K., et al., Adv. Funct. Mater. 2010, 17, 379-389; Dillon, A. C. et al., Thin Solid Films 2008, 516, 794-497]
Hence a redesign from the existing multi-component assembly to a completely new design of 3D nanoarchitectured electrodes with inter-penetrating or conformal assembly, [Long 2002; Cheah, S. K., et al., Nano Lett. 2009, 9, 3230-3233; U.S. patent application Ser. No. 11/372,286, entitled “Electrodeposition of a Polymer Film as a Thin Film Polymer Electrolyte for 3D Lithium Ion Batteries,” filed Aug. 27, 2007, inventors Madou M. J., et al.] separated by a thin electrolyte/separator will be essential to meet both energy and power requirements.
Nanostructured electrode materials due to their high surface area and superior electronic conductivity can be considered as potential candidates for the construction of 3D batteries. [Aricò 2005; Reddy 2009] The majority of the prior research efforts in 3D designs have been limited to the microstructured (˜40 μm pore size) battery architecture. [Golodnitsky 2006; Nathan 2005] Amongst the several methods available for synthesis of nanowire electrodes, template assisted synthesis has been shown to be a simple and versatile technique with excellent control over nanowire dimensions, [Hurst, S. J., et al., Angew. Chem. Int. Ed. 2006, 45, 2672-2692; Chong, F., et al., Chem. Mater. 2007, 20, 667-681] Conformal coating of electrode materials around nanostructured current collectors pioneered by Simon and co-workers [Taberna 2006; Bazin, L., et al., J. Power Sources 2009, 188, 578-582], have shown fast rates of charge and discharge maintaining high energy densities.
However, achieving uniform coatings of separator/electrolyte units around nanostructured electrode materials has been challenging and the reports addressing the same are limited. [Long, J. W., et al., Nano Lett. 2003, 3, 1151-1161 (“Long 2003”); Rhodes, C. P., et al., J. Phys. Chem. B 2004, 108, 13079-13087 (“Rhodes 2004”)] One method which is gaining focus is the self limiting electrodeposition of non-conducting polymers such as Poly(phenylene)oxide (PPO) around nanostructured electrodes. Long 2003; Rhodes 2004. The above method of coating polymers by electrodeposition requires extremely inert conditions to attain reliable conformal coatings around the electrode material. The other simple, established technique of coating polymer layers on electrode materials is by spin/drop coating. [Dewan, C., et al. J. Power Sources 2003, 119, 310-315; Pushparaj, V. L., et al., Proc. Nat. Acad. Sci. U.S.A. 2007, 104, 13574-13577] Such polymer coatings not only serve as the separator/electrolyte functionality but also could help in controlling and forming stable solid electrolyte interphase (SEI) film formation on the high surface area nanostructured electrodes. [Fu, L. J., et al., Solid State Sci. 2006, 8, 113-128 (“Fu 2006”); Balbuna, P. B., et al., Lithium-Ion Batteries Solid-Electrolyte Interphase, Imperial College Press, London 2004; Guo, K., et al., J. Power Sources 1997, 68, 87-90] SEI film formation and stability influence irreversible capacity loss and cycling characteristics of Li ion battery electrodes. [Fu 2006; Winter, M., et al., 1999, 45, 31-50 (“Winter 1999”)] The use of appropriate electrolyte for the active material, carbon coatings/composites, surface modification of electrode have included some of the efforts to address this issue. [Fu 2006; Ulus, A. et al., J. Electrochem. Soc. 2002, 149, A635-A643; Kim, T.-J., et al., Electrochim. Acta 2004, 49, 4405-4410; Lee, K. T., et al. J. Am. Chem. Soc. 2003, 125, 5652-5653] Hence there exists a need for innovative approaches to control the SEI formation on high volume expansion intermetallic [Stjerndahl, M., et al., Electrochim. Acta 2007, 52, 4947-4955 (“Stjerndahl 2007”); Ehinon, K. K. D., et al., Chem. Mater. 2008, 20, 5388-5398 (“Ehinon 2008”)] based electrode materials.